The present invention relates to an optical fiber amplifier having variable gain to be used in particular in a WDM network, and also to a WDM network and a method of amplifying WDM light signals.
Optical fibers are presently widely used for communicating information such as in large telecommunication systems, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. Of course, there is a desire in the existing telecommunication networks to use the available optical fibers in their networks as efficiently as possible, in particular for communication over long distances, since such fibers obviously have high installation costs. By introducing wavelength division multiplexing WDM in existing communication systems using optical fibers and in new communication systems to be built a plurality of individual wavelength channels can be transmitted on the same optical is fiber and thus the information transmitted over the fiber can be multiplied. In wavelength division multiplexing a plurality of optical signals, each on a separate wavelength channel, are simultaneously, in parallel to and independently of each other, transmitted on an optical fiber.
In optical fiber networks for example for long distance communication there may be a need for amplifying and/or regenerating the optical signals. Such amplification can of course be achieved by a repeater built in a straight-forward way, including components converting the optical signals to electrical signals, amplifying the electrical signals and converting the electrical signals to optical signals. For WDM signals this will require one optoelectrical and one electrooptical converter per wavelength channel used in the WDM transmission and also one filter or demultiplexer for filtering out the different wavelengths in the incoming signal. This will obviously be very costly and also results in reliability problems owing the large number of components, both electronic and optical, which are required.
Another type of amplifier comprises optical fiber amplifiers based on optical fibers doped with rare-earth metals, primarily erbium-doped fiber amplifiers. Such amplifiers have great advantages when used in optical fiber systems owing to e.g. their compatibility with the optical fibers and their high gain, and they are in particular advantageous when used in wave-length multiplexed transmission systems, since they are capable of simultaneously amplifying a number of WDM channels and only require a limited amount of electronic components. The basic design of an erbium-doped fiber amplifier includes one length of an active, erbium-doped optical fiber, connected at its input end to the output of a 2-to-1 optical coupler, the coupler receiving on one of its inputs the signal to be amplified and on the other input more energetic light providing the power for amplifying the signal. This more energetic input light is called the pump light and is obtained from an optical power source, called the optical pump. The pump light has a shorter wavelength than that of the signal and is generally more energetic and is capable of lifting erbium ions from lower energy states to higher energy states in the erbium-doped fiber. Light is then generated in the fiber when the ions return to lower energy levels.
In order to achieve the best possible transmission properties the power of all wavelength channels must on each considered point along the whole transmission path be kept equal to each other. In particular it is generally a strict requirement that the optical transmission must perform equally well irrespectively of which channels are present at each point and of the number of channels which are present at each point. Hence, it is usually optimal to have a constant output power per optical channel in an optical amplifier included in optical network. Further, the gain of an optical amplifier must be maintained in the cases where wavelength channels are added to or dropped on the input line of the optical amplifier. Otherwise this will cause, in an optical fiber amplifier which is normally operated in a saturated condition, implying that it has an approximately constant output power independent of the input power, transients in the power levels of the incoming channels to which a channel is added or of the remaining channels after dropping a channel respectively.
Thus, the output power of an amplifier employed in a network must be controlled as efficiently as possible. Prior methods are disclosed in Swedish patent No. 506 403, Swedish patent application No. 9603336-0 corresponding to published International patent application WO98/11682, the International patent application No. PCT/SE98/0255, and the International patent application No. PCT/SE99/00556 corresponding to Swedish patent application No. 9801159-6, filed Apr. 1, 1998. A commonly used method of controlling an optical fiber amplifier is to vary the optical pump power by regulating the current flowing in the pump laser diode. However, due to internal delays in such an amplifier it is difficult to construct an efficient and fast control using only a feedback loop, see e.g. the cited International patent application No. PCT/SE99/00556. Still, it is a well-known fact that a feed forward loop can be very efficient in providing a fast control in a regulated system provided that the transfer function of the system is accurately known. The gain of an optical fiber amplifier is not a linear function of the pump power and furthermore, the optical pump power supplied from a pump laser is not a linear function of the current supplied to the laser, the bias current.
A pump laser diode is a very fast non-linear device and thus a local feedback loop can be used to regulate the pump power. The signal from the monitor photodiode which is normally available at the rear facet of the laser diode within a pump laser module or package can be used as a feedback signal for this regulating loop. Still this monitor signal is not fully proportional to the optical pump power fed to the optical fiber amplifier.
An optical amplifier is disclosed in U.S. Pat. No. 5,374,973 comprising the conventional components such as a pump power monitor and an output power feedback loop. No provisions are made to compensate for non-linearities in the essential elements of the amplifier, i.e. in the amplifying optical fiber, the pump laser and the monitor diode.
As disclosed in the cited Swedish patent application No. 9603336-0 a good amplifier control can be achieved by combining feed forward and feed back loops. The feed forward loop includes a non-linear element using, in the disclosed embodiment, A/D-conversion, a stored table of numeric values modifying the control digital signal and then D/A-conversion. However, it is difficult to derive the optimal characteristics of this non-linear element, i.e. the table, from measurements on the amplifier. It may also be desired to implement them in hardware in the loop avoiding the A/D- and D/A-conversions. Furthermore, the accuracy of the control is limited by this model and the non-linearities of the pump laser, resulting in a less than optimal performance of the amplifier.
It is an object of the invention to provide an optical amplifying device having a gain which can be accurately controlled and in particular be maintained at an accurate constant value.
It is another object of the invention to provide an optical amplifier having a controllable accurate gain which can be readily built in hardware.
The problem solved by the invention is thus how to construct an optical amplifier which has a constant gain irrespectively of the input signal power, where the amplifier has a construction suited to be built of hardware and the construction requiring only specific, definite measurements on the amplifier and its pump laser and pump laser monitor.
Thus, an optical amplifying device comprises an active fiber length, a pump laser included in a pump laser module also including a monitoring diode, and further two control loops, a feed forward loop and a feed back loop. In the feed forward loop the input power level is modified to a signal corresponding to or substantially agreeing with the pump power which maintains the desired gain, then adding or subtracting suitable signals to compensate for offsets. The pump power level obtained by the modifying is then converted to correspond to or agree with or to have a similar behaviour as the pump monitor signal which is used as the set value in the local pump laser regulating loop of feed back type. This second conversion can also be used for any control signal or control device providing a control signal to the pump laser irrespectively of the type of control used.
The advantage of the proposed control is that it provides a very fast and accurate control scheme for such an inherently non-linear and complicated device as an optical fiber amplifier. The parameters required for implementing the control scheme are directly given by making some definite measurements on the amplifier and its pump laser module.